1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method of monochrome or color, typified by copiers, printers, facsimile or complex machines thereof, which includes a transfer fixing member which bears transferred toner images, a heating unit which heats the toner image on the transfer fixing member and a pressurizing member which forms a transfer fixing nip with the transfer fixing member, and by which the toner image on the transfer fixing member is transferred and fixed simultaneously to a recording medium such as paper passing through the transfer fixing nip to record the image on the recording medium.
2. Description of the Related Art
Image forming apparatuses in which images are formed on image bearing members by means of developing units, the images on the image bearing members are transferred primarily to intermediate transfer members by means of primary transfer units, the images on the intermediate transfer members are further transferred secondarily to recording media by means of secondary transfer units and the images on the recording media are then fixed have been widely known. The image forming apparatuses which perform entire processes step by step currently predominate the market, however, image forming apparatuses which perform transferring and fixing steps simultaneously, that is, having a transfer fixing step such as the ones disclosed in Japanese Patent (JP-B) No. 3042414 or Japanese Patent Application Laid-Open (JP-A) No. 2004-145260 are also known.
In JP-B No. 3042414, a method for forming a transfer fixing nip by disposing a heat source 102 inside a driving roller 101 of an intermediate transfer member 100 and by pressure welding a pressurizing member 103 to the intermediate transfer member 100 as shown in FIG. 1 is proposed. It is a kind of method in which toner is heated before going through the transfer fixing nip and the heated toner is then transferred and fixed to a recording medium 104 from the intermediate transfer member 100 through the transfer fixing nip. The symbol 105 represents four image bearing members of each color and 106 represents primary transfer units for each image bearing member 105. By this method, secondary transferring from the intermediate transfer member 100 to the recording medium 104 is performed by the heat for fixing instead of electrostatic force. Moreover, it is possible to set heating time of toner longer.
In the method stated in JP-B No. 3042414, the intermediate transfer member 100 is also heated for the same time interval as the heating time of toner and in addition, the intermediate transfer member 100 is heated from inside to the whole member in a layer thickness direction. Because of this, when the intermediate transfer member 100 enters a primary transfer area, the image bearing members 105 are also heated by the heat of the intermediate transfer member 100, resulting in problems such as toner fixation.
The image forming apparatus stated in JP-A No. 2004-145260 is equipped with a transfer fixing member in which an image formed in a traveling direction of an intermediate transfer member for preventing heating of the intermediate transfer member is transferred, a heating unit which heats the image on the transfer fixing member and a pressurizing unit which forms a transfer fixing nip with the above transfer fixing member and the image is transferred and fixed from the transfer fixing member to a recording medium tertiarily after the image is transferred and fixed from the intermediate transfer member to the transfer fixing member.
Meanwhile, it is common to use electrically chargeable fine particles consisting mainly of resin which is called toner as a member for making up an image in these techniques.
For the conventional image forming apparatus, image quality tends to be degraded in a step of transferring to a recording medium. Paper, the mainly used recording medium varies from regular paper to heavy paper and its surface property also varies from high quality to irregular paper. Specifically in the case of paper with a rough surface property, microscopic gaps are formed due to an intermediate transfer member which cannot follow the surface property of the paper, an abnormal discharge occurs in the microscopic gaps, an image is not transferred normally and tends to be indistinct.
In contrast, because transfer and fixing are performed simultaneously in the image forming apparatus which contains a transfer fixing step such as above, the degradation of image quality is least likely to occur even when paper with a rough surface property is used. This is because heat is added simultaneously during transfer, toner is softened and melted by heat to become a viscoelastic block-shaped mass, making it easier for the image even in the microscopic gap of paper to be transferred. Because of the advantage such as above, the image forming apparatus having a transfer fixing unit can be said to be suitable for forming images of high quality.
However, a transfer fixing ratio is low and graininess is inappropriate for the highlight area produced by these fixing methods. In other words, it is known that the toner is unlikely to be shifted to a recording medium sufficiently and images are not improved and sometimes may be degraded compared to normally-operated electrostatic-transfer methods. Furthermore, it has been found that when energy added during transfer fixing is increased in order to improve the transfer fixing ratio and image quality of a highlight area, the transfer fixing ratio of the highlight area becomes appropriate even in the highlight area, however, problems of irregularity in fixing and glossiness may occur in high-density areas where a toner amount is high due to excessive fixation of the toner.
Toner viscosity from molten condition to transfer fixing is regulated in JP-B Nos. 3042414 and 3021352 for improvement. However, when a nip time, the time it takes for a recording medium to pass through a fixing nip is set at 30 ms or less for achieving high-speed printing, if there is a tiny difference in viscosity in the toner image, fixation to the recording medium is inhibited and a sufficient transfer property in order to produce a sufficient anchoring effect of a toner image on a recording medium cannot be obtained and as a result, degradation of highlight was unavoidable.